This invention concerns variable speed gas turbine engines of a type that can be especially useful for driving electric generators or vehicles and in other applications where it is preferred to have a moderate power output, small size and high response speed. The efficiencies of variable speed turbines vary according to a number of variables, including engine speed. In prior art gas turbines of this type, when the turbine speed decreases and the power output remains unchanged, such as in the case of where a vehicle travels up a slope, the compressor speed will rise, its power will increase, and much more fluid than necessary will be supplied to the turbine. The engine consequently becomes "overcooled", the cycle temperature drops, and contraction of metal parts occurs. As a result, turbine power output decreases, and efficiency is reduced.
When turbine speed remains unchanged, and the power output decreases, such as in the case of where a vehicle travels down a slope, compressor speed decreases to a great extent and the turbine experiences a shortage of fluid. The engine thus becomes "overheated," which poses risks to turbine engine components due to excessive metal expansion.
During overcooling, the compressor turbine has an excess of power that floods the turbine with fluid. During overheating, there is a shortage of power at the compressor turbine, and the turbine receives less fluid than it needs, which leads to overheating. Thus, temperature is a critical parameter to control in engines of this type. Both phenomena can be counteracted by controlling fluid flow to the compressor turbine or by controlling fluid flow to the turbine. In both cases, such control is accompanied by losses.
In our pending application Ser. No. 09/161,170, filed Sep. 25, 1998, I disclose a gas turbine engine having a turbine mounted downstream of a compressor and a compressor turbine mounted downstream of the turbine for driving the compressor. The compressor turbine has a rotor disk that is mechanically coupled to the compressor and rotates in a direction opposite to the direction of rotation of the turbine rotor disk. A heat exchanger has a first circuit connected to the compressor turbine and a second circuit connected between the compressor and the turbine. An electric load for consuming a fraction of power produced by the compressor turbine includes an electric generator that is mechanically coupled to the compressor turbine. The electric load controller varies the electric load consumed based on temperature reading from the engine, thus changing the amount of power produced by the compressor turbine. This load is varied in response to changes in temperature and other operating characteristics in order to hold such characteristics within desired ranges. The above-described system allows the temperature at the exhaust of the compressor turbine to be kept stable. The stable temperature at the exhaust assures stable operating parameters and temperature conditions in the flow duct of the gas turbine engine. This enhances efficiency and reliability of the gas turbine engine and prolongs service life. However, the temperature in the combustor is about 1900K. It should be noted that formation of nitrogen oxides during fuel combustion starts from 1800K. Nitrogen oxide emissions are very harmful, and are subject to strict control in many jurisdictions. Further, to control CO levels, emissions of which are also of environmental concern, it would be desirable to have a maximum combustor temperature of 1500K. To achieve this, a very lean fuel and air mixture must be used, with an excess air factor of about 2.2. This is practically unachievable.
This emissions problem was addressed in a gas turbine engine (Electric Vehicles, PT58, 1997, Ed. By Ron Sims and Bradford Bates, Society of Automotive Engineers, Inc., Warrendale, Pa., p. 143-145) having a compressor, a turbine, a heat exchanger for heating air from the outlet of the compressor with the exhaust gases of the turbine before supplying the air to a combustor and a catalytic combustor. The catalytic combustor assures combustion of a very lean fuel and air mixture having an excess air factor of 6 to 8. Combustion with this excess air factor occurs at 1050 to 1100K, thus ruling out the formation of nitrogen oxides. The catalytic combustor is made as a catalytic thermochemical reactor containing a catalyst bed in a special screen casing and having a special heater for preheating the catalyst before starting the gas turbine engine. The reactors of this type require much space and have a complicated design. In addition, a certain time is required for heating the catalyst before starting the gas turbine engine. Another disadvantage of this gas turbine engine is contamination of the catalyst, which is aggravated when normal fuel containing sulfur and other impurities is used. As the catalyst becomes contaminated, its catalytic efficiency decreases, the combustion temperature rises, and the NO.sub.x level in emission also increases. It should be also added that catalysts are consumable materials and will add to the cost of operation of the gas turbine engine.
Another attempt to increase efficiency of a gas turbine engine with the use of a thermochemical reactor (Nosach N. G., Energiya topliva [in Russian] AN Ukr. SSR, Institut tekhnicheskoy teplofiziki. Kiev., Naukova Dumka, 1989, p. 78) involves mixing fuel with combustion products from the exhaust of the turbine after passing through a heat exchanger. The mixture of fuel with combustion products, which contain CO and water, is compressed in a special compressor and is then fed to a thermochemical reactor that is heated to about 900K with the exhaust gases from the turbine under normal operating conditions (full speed). The fuel and exhaust gases react in the thermochemical reactor and decompose into CO and hydrogen, the quantity of the combustible material increases, and the overall amount of energy available in the fuel increases by 40 to 44%. The resulting fuel from the thermochemical reactor is supplied to the combustor, which also receives air from a compressor of the gas turbine engine that is heated with the exhaust gases of the turbine. This cycle of the gas turbine engine should have resulted in an improved efficiency of the gas turbine engine. As the reaction of fuel conversion in the thermochemical reactor can occur only starting from the temperature of about 800K, this reaction cannot take place under low-power operating conditions of the gas turbine engine when the exhaust gas temperature can be as low as 500K. It should be also noted that a temperature of at least 1000K is required for the complete reaction to occur. In addition, there can be no oxygen in the mixture that is compressed in the special compressor before conversion in the thermochemical reactor. It is known that oxygen is always available in the combustion products (up to 10%). This oxygen will immediately oxidize the fuel in the compressor up to self ignition. As a result, a part of the combustible components of the fuel will be burned before reaching the thermochemical reactor. It will be understood that in a gas turbine engine that have to be used in vehicles or in electric generation sets operating under variable loads, this method cannot be used to the full advantage, and the desired increase in the overall efficiency cannot be assured. It should be also noted that the fuel obtained after conversion in the thermochemical reactor is burned in a conventional combustor with a combustion temperature of about 2200K, forming NO.sub.x.
These disadvantages are eliminated in a gas turbine engine according to the invention as described below.